1. Field of the Invention
The present invention relates to cardiac pacers, and more particularly, to a rate responsive cardiac pacer which automatically controls the speed with which pacing rate can be changed so as to provide a more physiological response to changing cardiac demand.
2. Description of the Prior Art
The rate responsive cardiac pacemaker is now acknowledged to provide a significant improvement over prior fixed-rate cardiac pacemakers. Indeed, rate response techniques have been applied to both single and dual chamber pacemakers, and are able to control pacing rate as a function of one or more measured parameters which are directly or indirectly related to cardiac demand. Such measured parameters include, for example, the physical activity or movement of the patient, right ventricular blood pressure and change of such blood pressure over time, venous blood temperature, venous blood oxygen saturation, respiration, minute ventilation and Q-T interval. A more recent advent is the employment of two or more sensors and the use of algorithms for obtaining an optimized sensor-heart rate correlation function. See, for example, U.S. Pat. No. 5,065,759, entitled "Pacemaker With Optimized Rate Responsiveness and Method of Rate Control," assigned to the same assignee as this application, and incorporated herein by reference. An advantage of the combined sensor approach is to provide a sensor response which accurately tracks cardiac demand both during fast time period episodes and during more stable steady state patient conditions. For example, the Q-T parameter is very reliable as a steady state indicator of metabolic demand, but does not respond quickly to fast changes, such as at the onset of exercise. On the other hand, the activity-type sensor, such as described in U.S. Pat. No. 4,485,813, issued to Anderson et al., and assigned to Medtronic, Inc., provides a faster response to changing patient conditions, even though it may not be as accurate an indicator of quiescent demand.
A problem that pacemaker designers have encountered, but not yet resolved fully satisfactorily, is that of translating sensor indications of desired pacing rate into actual changes in pacing rate. By way of example, at both the onset and termination of exercise the body's physiological response is known to be different from what accurate sensor signals would otherwise indicate. Thus, at the onset of exercise, the heart does not increase rate as a step function, but increases over a period of time and as a function of the degree of exercise. Likewise, following termination of exercise, the physiological heart rate decays in accordance with a certain decay function. Accordingly, there have been efforts to resolve the discrepancy between normal heart deceleration function at the end of physiological stresses such as physical activity, and the normal physiological decay function. See, for example, U.S. application Ser. No. 07/567,204, filed Aug. 14, 1990, Bennett et al., entitled "Rate Responsive Pacemaker and Pacing Method"; and U.S. patent application Ser. No. 880,877, filed May 11, 1992, "Work-Modulated Pacing Rate Deceleration," both assigned to Medtronic, Inc.
The inventor of this invention has also observed that the appropriate response of a pacemaker to changed sensor indication of heart rate is generally a function of patient history, and specifically recent heart rate (paced or natural). This is a general rule, and is the case not just at transitions from rest to exercise or exercise to rest. Thus, if a patient has already had a relatively high heart rate, e.g., 85 bpm, and the sensor indication calls for a still increased heart rate, the correct physiological response may be different from the situation where the patient starts with a lower heart rate, e.g., around 70 bpm.
As a consequence of the above observations, it is seen that there is a need for an improved pacemaker apparatus, and method of controlling the rate responsiveness of a pacemaker, for providing a more appropriately physiological response under conditions where accurate sensor information concerning cardiac demand is available.